By Richard Black
Environment Correspondent, BBC News website
Summer of 2003: Unusually and uncomfortably hot in Europe
A new study shows that during the 2003 heatwave, European plants produced more carbon dioxide than they absorbed from the atmosphere.
They produced nearly a tenth as much as fossil fuel burning globally.
The study shows that ecosystems which currently absorb CO2 from the atmosphere may in future produce it, adding to the greenhouse effect.
The 2003 European summer was abnormally hot; but other studies show that these temperatures could become commonplace.
In some parts of Europe, 2003 saw temperatures soaring six degrees Celsius above normal; hot enough that estimates of the deaths which it caused run into the tens of thousands.
It was also significantly drier than usual; and these two factors appear to have had a major impact on plant growth.
Up the tower
"The data we used mainly comes from a set of 18 flux towers which are set up across Europe," said Andrew Friend, from the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) in Gif-sur-Yvette near Paris, whose team published their study in the scientific journal Nature.
The towers, managed through a project called CarboEurope, measure the flow of carbon dioxide, water and energy between the atmosphere and the ground; most are set up in forests.
Harvests of some important crops declined in the heatwave
"About half of the mass of a plant is carbon; so by measuring the flow of CO2 into the plants, we can see how well they're doing," Dr Friend told the BBC News website.
The result coming from the 18 sites was that during 2003, plants took up less CO2 from the air and grew more slowly - a finding corroborated by satellite measurements of the area under leaf.
So much for natural ecosystems, but what about farmland?
Here, the researchers drew on data from the UN Food and Agriculture Organisation, which showed a fall in European crop yields during the 2003 summer.
Putting all the data together, the headline figure is that, overall, European lands were 20% less productive than during an average year.
"We expect that many crops will be affected by high temperatures, especially during critical phases of development such as flowering," said Professor Julia Slingo, from the Natural Environment Research Council's Centre for Global Atmospheric Modelling in the UK, who recently organised a Royal Society seminar on food crops in a changing climate.
"This study found that crops reaching maturity in August were particularly badly affected; some of the fall-off could be related to water stress, but could also have been related to high temperatures during flowering," she told BBC News.
"The heatwave also led to higher levels of ozone at ground level, and that can have damaging effects on plants."
Saint becomes sinner
The really surprising finding came with the calculation that during the heatwave, European plants and their ecosystems were putting more carbon dioxide into the air than they were absorbing.
"In the past we expected that climate change would benefit European ecosystems because growth tends to be limited by the short growing season," said Andrew Friend, "but this analysis hadn't taken into account the possibility of extreme events.
"The conclusion of our study is that this extreme event meant a loss of carbon across Europe - a loss which undoes many years of net uptake."
Plants can absorb and emit carbon dioxide and oxygen; the process of respiration takes oxygen in and releases CO2, whereas in photosynthesis, the reverse happens.
Other parts of the ecosystem such as soil bacteria can also contribute to the overall flow of these gases to and from the atmosphere.
During an average year, the net effect is that European plants absorb around 125 million tonnes of carbon (MtC).
But in 2003, according to this analysis, they released 500 MtC to the atmosphere.
By comparison, global emissions from burning fossil fuels amounts to about 7,000 MtC; by giving rather than taking, European plants were adding about 10% to the global total.
"This shows that short-term climatic events such as the 2003 heatwave occurring over regional areas like Europe can have major effects on the climate globally," commented Julia Slingo.
The heat to come
The wider context for all this is a study published last year suggesting that summers as hot and dry as that of 2003 will become commonplace as the global climate changes.
"We concluded that on a middle-of-the-road scenario for emissions - assuming we don't do very much to combat climate change - temperature heatwaves as high as the one in 2003 would be occurring every other year by middle of this century," said Dr Myles Allen of Oxford University.
"By the end of the century, 2003 would be a cool year."
Plants could of course adapt to the changing climate, meaning that the switch from net absorption of CO2 to net production might not happen.
But, said Andrew Friend, this finding may be a sign of things to come.
"In the tropics, where it's already warm, higher temperatures are predicted to increase the flux of carbon from plants to the atmosphere," he said.
"We have generally assumed that in northern systems, we would see increased carbon uptake; but that might not be the case."